Using Radioiodine Speciation to Address Environmental Remediation and Waste-Stream Sequestration Problems at the Fukushima Daiichi Nuclear Power Plant and a DOE Site

Abstract

Iodine-129 is one of three key risk drivers at several DOE waste management sites. Natural organic matter (NOM) is thought to play important roles in immobilization of aqueous iodide (I-) and iodate (IO3-) in the environment, but molecular interactions between NOM and iodine species are poorly understood. In this work, we investigated iodine and carbon speciation in three humic acid (HA)-I systems using I K-edge XANES and EXAFS and C K-edge XANES spectroscopy: 1) I- in the presence of laccase (an oxidase enzyme) and a mediator, 2,2?-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) in pH 4 buffer, 2) I- in the presence of lactoperoxidase (LPO) and H2O2 in pH 7 buffer, 3) IO3- in pH 3 groundwater. The oxidase and peroxidase systems were less effective than the laccase-ABTS mediator system at oxidizing I- to I2 or hypoiodide (HOI), resulting in I- uptake by HA increasing from 0.4 to 2.9 mg/g in the oxidase and peroxidase systems to 13.5 mg/g in the laccase-ABTS mediator system. IO3- was abiotically reduced to I2 / HOI. Pathways for HA iodination include covalent modification of aromatic-type rings by I2 / HOI or iodine incorporation into newly formed benzoquinone species arising from oxidation of phenolic C species. This study improves our molecular understanding of NOM-iodine interaction and describes the important role that mediators may play in the enzymatic reactions between iodine and NOM.

KEYWORDS: Enzyme, Humic acid, Iodine immobilization, I K-edge XANES and EXAFS, C K-edge XANES